An edge-lit type plane light source device is used as a backlight for a liquid crystal display device. The edge-lit type device includes light sources such as LEDs (Light Emitting Diodes) along an edge of the light-emitting surface of the plane light source device and is configured to guide light emitted from the light sources to the light-emitting surface by a plate-shaped light guide (called a light-guiding panel). The edge-lit type plane light source device can be relatively easily reduced in size/thickness and therefore has been widely used in small size electronic equipment such as a smart phone.
An edge-lit plane light source device may be encountered with a failure related to luminance unevenness attributable to various causes such as a defect in a mold for a light-guiding panel or poor molding thereof and shifting during assembly. One such failure is an extremely brighter part than standard luminance appearing at an end excluding a part provided with light sources. (Herein, the failure of this kind will be referred to as a “bright edge”.)
At present, inspection of failures of this kind actually depends on visual sensory inspection carried out by a person (an inspector). Therefore, the inspection procedure may take time and trouble and can be costly or the results of the inspection depend much on personal skills, and there has been a demand for automation and objectification (quantification) of the inspection.
Note that PTL 1 proposes a method for automatically inspecting a liquid crystal panel for luminance unevenness (referred to as a “spot defect” in the document) by image processing though the document does not concern inspection of a plane light source device. However, when the inventor attempted to apply the method disclosed in the document to inspection of a bright edge, the result was not comparable with human sensory inspection.